1. Field of the Invention
The present invention relates to superconductor analog-to-digital converters. More specifically, the present invention utilizes Single Flux Quantum and time-interleaving techniques to create a novel modulator for delta and delta-sigma analog-to-digital converters.
2. Background Information
An analog-to-digital converter (ADC) samples an analog signal at discrete time intervals and quantizes the amplitude of each sample. It outputs a digital code that represents each quantized amplitude. Traditionally, the analog signal is sampled at the Nyquist rate. The Nyquist rate is twice the frequency of the highest frequency component of the supplied signal and is the minimum rate of sampling required to digitally capture the signal.
The quantizing process inherently introduces error because of the uncertainty in determining the amplitude of a signal that falls between two quantizing levels. This quantization error becomes noise. Quantization error can be minimized by decreasing the spacing between quantizing levels and increasing the number of levels. This approach can place stringent demands on the analog components used, particularly since there are other sources of noise as well as interference. It is difficult to realize desirable ADC""s in very large scale integration (VLSI) technology. Oversampling is one approach to dealing with this problem.
When a signal is oversampled the sampling rate is much higher than the Nyquist rate. This approach is beneficial because when a signal is sampled at a particular frequency, the noise power is spread out in a frequency range from zero to half the sampling frequency. When sampling at the Nyquist rate, this particular noise power is spectrally distributed over the signal bandwidth. Using oversampling, the same noise power is spectrally distributed over a bandwidth greater than the signal bandwidth. Therefore, for a significantly larger sampling frequency, the noise power within the signal band is significantly decreased. The demands on the analog components of an ADC can therefore be relaxed because more noise can be tolerated. Oversampling takes advantage of the fact that VLSI technology is better suited for providing fast digital circuits than for providing precise analog circuits.
Most early work on oversampling focused on delta modulators, which are based on generating and subtracting from the input signal the quantization error of a low-resolution quantizer placed in the forward path of a feedback loop. As shown in FIG. 1, high-speed oversampling within ADC 10 takes place within the modulator 12 with the use of high speed clock 28. A digital filter 14 smoothes the output of the modulator which attenuates noise, interference, and high-frequency components before the digital signal is sampled at the Nyquist rate, using the clock 26, by the register 16 for digital output to 24 as n-bit words. Within the modulator, digital code output is reconverted to an analog signal via a digital-to-analog converter (DAC) 22 prior to subtraction from the input signal. The resulting system predicts and corrects the next quantization error value. A filter placed in the return section of the feedback loop, shown as part of the integrated feedback loop filter block 20, causes the input signal at 30 and the quantization error to be filtered by the inverse of the loop filter transfer function. The subtraction of the feedback signal from the input signal as well as the filtering function both taking place within the integrated feedback loop filter 20. In the case of delta modulation the filtering is done prior to the subtraction.
Later work on oversampling focused on delta-sigma modulators because the circuits are more robust. These modulators are based on the principle of generating and subtracting the prior quantization error of the low resolution quantizer 18 from the input signal such that the output contains the original input signal plus the first difference of the quantization error. In this case, the filter is placed in the forward path of the feedback loop so the subtraction is done prior to the filtering within the integrated feedback loop filter 20.
The delta-sigma approach has the effect of leaving the input signal unchanged while significantly shifting the spectral distribution of the noise to outside the signal band, and is therefore called noise-shaping modulation. The noise within the input signal band is substantially shifted to outside the band. This effect contrasts with that of the noise predicting approach of delta modulation that changes the spectral distribution of the input signal while leaving that of the noise unchanged. However, the basic concept underlying both types of modulators is the use of feedback for improving the effective resolution of a coarse quantizer.
Due to the high sampling rates required by the oversampling approach, time-interleaving of the quantizer and DAC was introduced to allow conversion of larger bandwidth signals. The operation cycle of a quantizer consists of two phases: sample and hold. The quantizer is sensitive to the input signal only during the sampling phase. During the holding phase the quantizer calculates a digital code or more exactly compares the stored signal with a threshold value. The duration of the sampling phase defines the bandwidth and accuracy of the ADC and it should be as short as possible. The duration of the holding phase is also important because it defines the minimum clock period and therefore the oversampling ratio. Since the holding phase is relatively long, interleaving allows a higher sampling rate by using several quantizers that sample during each others holding phases.
Typical commercially available modulators allow for a bandwidth of a few kilohertz. Even with time-interleaving, the need for very high performance ADC""s in the immediate future cannot be met by the semiconductor industry, therefore superconductor technology is being explored as a viable alternative.